This invention relates to an electrodialytic process for recovering acids from materials comprising acid and soluble salt. More particularly, the invention is directed to the recovery of mixed acids, such as HF and HNO.sub.3, from a spent process material, such as spent pickling liquor, which comprises free acid and soluble salt. The process for recovering acid from such materials employs electrodialysis to recover the free acid from the material and electrodialytic water splitting to regenerate acid from the soluble salt.
Pickling baths, for example, are employed to remove the scale, oxides and other impurities from metal surfaces such as stainless steel. These baths comprise inorganic acids such as hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid, and hydrofluoric acid, and commonly are mixtures thereof. In use, the bath gradually becomes exhausted due to the reaction of the acids with the oxides, scale, etc. Eventually, the bath is converted to a spent solution comprising mixed salts and some free (unreacted) acid. This spent solution must then be disposed and the acid must be replaced at a cost which can be very substantial.
The economic impact of disposing of the spent process materials can be as expensive as the cost of replacing the acid bath. Moreover, the environmental impact of disposing of these materials is of significant concern. One method of disposing of spent pickling liquor requires neutralizing the liquor with lime to precipitate the metal values and fluorides, and then landfilling the solid waste, with the remaining solubles being discharged after further treatment. However, this disposal process is very expensive and, because of the toxicity of the especially if it contains fluoride ions, significant environmental damage can occur if it is improperly disposed.
Processes are known for the recovery of free acid from materials containing the same. They include (a) diffusion dialysis, (b) acid retardation using ion exchange resins, and (c) "the Ruthner process." Diffusion dialysis is the process of removing free acid from a liquid by transporting acid from a compartment containing the liquid across a membranes to adjacent compartments containing water, where the transport driving force is the concentration gradient of acid across the membrane. Acid retardation is the process of absorbing free acid from the material by use of an ion exchange resin, removing the ion exchange resin containing the acid from the material, and then washing the ion exchange resin with water to desorb the acid. "The Ruthner process" is the process of isolating free acid from a liquid via an evaporative crystalization process.
Each of the above mentioned processes has major shortcomings. With diffusion dialysis, acid recovery diminishes as the concentration gradient is reduced. With acid retardation using ion exchange resin, acid recovery is limited by the absorption and desorption capacity of the resin. Moreover, both processes have the added problem of disposing of the remaining acid-depleted salt stream. With "the Ruthner process" the regenerated acid normally contains five to ten times the metal content as the acid produced by the above mentioned processes. Moreover, the Ruthner process is energy intensive and highly corrosive to the process equipment.
Electrodialytic water splitting processes for regenerating acids and base from salts are known. For example, in U.S. Pat. Nos. 4,082,835 and 4,107,015, processes are disclosed for regenerating scrubbing solutions used in stripping SO.sub.x from flue gases by feeding salt-containing product solutions from the stripping step through an electrodialytic water splitter. Also, in U.S. Pat. No. 4,504,373, a process is disclosed for regenerating a dilute sulfuric acid solution for use in the processing of rayon from a spent rayon spin bath containing a sulfate salt by subjecting the salt to electrodialytic water splitting.